Over 90% of Americans drink caffeinated beverages daily. Caffeine appears to fulfill several criteria for a drug of dependence; i.e., it can produce: 1) adverse effects (e.g. arrythymias, insomnia, etc.), 2) physical dependence (e.g. withdrawal symptoms of headache and fatigue), and 3) behavioral effects that might be rewarding (e.g. decreased fatigue and improved performance). However, whether caffeine fulfills the most crucial criteria for dependence potential - ability to serve as a reinforcer - is debatable. For example, whether coffee drinkers will consistently self-administer caffeinated coffee in preference to decaffeinated coffee has only begun to be studied. The ability of a drug to function as a reinforcer is highly dependent on pharmacological, environmental and behavioral conditions. This application proposes six parametric studies to determine conditions under which caffeine will serve as a reinforcer. We will test dose (25, 50, 100, & 150 mg), present history of drug use (heavy users, light users & abstainers), vehicle (coffee, non-coffee beverage & capsules), instructions (told beverage does or does not contain caffeine), fatigue (after normal sleep vs. after sleep deprivation) and method of testing (concurrent access vs. exclusive choice tests). These studies will help NIDA deliver rational public information about whether coffee drinking is a drug dependence. They will also help devise treatments for those who need to abstain from caffeine but have difficulty doing so. Finally, the studies will test whether commonalities across forms of substance abuse can be extended to coffee drinking.